There are a number of matters of general or particular concern in the design and operation of oven systems for curing coated products of all sorts. Oven systems are used for curing coated product such as fabric, coated discrete articles such as automobile or appliance parts which are carried through the oven by a conveyor, and even products such as coated wire or mesh. More particularly, coated coiled metal strip of steel or aluminum are cured in oven systems which may be very large and operate at high speeds and high temperatures, and in any event where the coating material is heat-curable and releases flammable or volatile solvent from the coated product as it travels through the oven line. Not only must the oven system be operated very safely, with no possibility of the buildup of a potentially explosive mixture within the oven, it must also be operated with reasonable cost efficiency and without contributing to environmental pollution.
Thus, it has long been realized that volatile solvent released from the coated product as it is being cured must be incinerated; and it has been recognized that if the volatile solvent is to be incinerated, then its heat contribution should be used for curing coated product which follows it through the oven.
It is also been well known that, in order to avoid any spillage of solvent and potentially explosive fumes from the oven, there must always be an indraft at both ends of the oven line.
This has created yet other problems, however. That is, if there is to be an indraft into the oven, which comprises a flow of cold air, and there is to be a flow of heated air into the oven line to maintain it at its elevated temperature, then the fan inducing that flow must handle a volume flow which is essentially twice the indraft volume at elevated temperatures; and the stack output is at very high temperatures. A typical oven schematic for such a system is discussed in greater detail hereafter.
Where a principal concern has been to provide a solvent free exhaust, WHIKE et al in U.S. Pat. No. 4,217,091 dated Aug. 12, 1980 have provided a method of dividing the over line into a plurality of zones in which solvents are vaporized and entrained, where recirculation of solvent-rich vapours from zone to zone is accomplished. That oven system, however, requires a number of zone incinerators, and a number of fans, thereby being a relatively high capital cost system.
JAMALUDDIN, in U.S. Pat. No. 4,240,787 issued Dec. 23, 1980, provides a similar kind of system, again requiring substantial capital cost and numerous zone incinerators and fans. A disadvantage of both of those systems, however, is the fact that combustion takes place directly within the ovens.
CROMP, in U.S. Pat. No. 3,737,280 issued June 5, 1973, has another approach where a number of burners, some of which are fuel fired and at least one of which is a solvent fired burner, are provided at the bottom of a stack. Some of the hot air exiting from the stack is diverted through the oven, mixed with fresh air, and some of the solvent-laden air coming off the oven is fed back to the solvent burner. The system of that patent reduces the fuel costs, but still requires considerable contribution by fuels other than solvent released from the coating material.
An oven line where both heated and cool gases are circulated to a plurality of oven chambers is that which is taught by SCHREGENBERGER in U.S. Pat. No. 4,326,342 issued Apr. 27, 1982. However, in this patent, the gas flow control is pressure related.
Another patent of interest is ELLISON et al U.S. Pat. No. 4,206,553, issued June 10, 1980. That patent teaches an oven and a method of operating the oven, where entrained vaporized solvent is removed from one or more oven zones but, rather than being fed off line to a separately disposed burner, they are fed to different oven zones where they are incinerated. Thereafter, the solvent-depleted vapours are fed to another zone and mixed with oven gases circulating in that zone so as to maintain a stable solvent vapour content and operating temperature in that other zone.
WILKINSON, in U.S. Pat. No. 3,757,427 issued Sept. 11, 1973, teaches an oven and a method of drying a solvent containing coating on the surface carried through the oven by exhausting the gases from the oven, and then splitting the exhaust into two streams, one of which is incinerated and the other of which is re-cycled directly back into the oven. One of the streams leaving the oxidizer or burner is vented to atmosphere, and the other is again passed into the oven together with the recycle stream of unburnt gases. In this system, some fresh air is drawn into the oven line, usually through a heat exchanger which is heated by the waste combustion gases, where the heated fresh air mixes with the incoming return stream of combustion gases, all for the purpose of maintaining safety limits and generally energy efficient operation.